1. Field of the Invention
The present invention relates to an active matrix OLED display, and in particular, to power circuits that compensate temperature variations when driving the display.
2. Description of the Related Art
FIG. 1 shows a pixel circuit of a conventional active matrix OLED display. A capacitor 104 is coupled to the gate of a driving transistor 106, and an OLED 102 is coupled to the drain of the driving transistor 106. The source of the driving transistor 106 is coupled to a terminal VDD, and the other end of the OLED 102 is coupled to a terminal VSS. The pixel circuit shown is an abstract concept, in which the driving transistor 106 may be a PMOS or an NMOS, and the OLED 102 may also be coupled to the terminal VDD and the driving transistor 106. Thousands of variations of detailed implementations are present and known to the art. The major principle is that the capacitor 104 determines brightness of the pixel circuit, and the OLED 102 illuminates in response to the current flowing from terminal VDD to terminal VSS controlled by the driving transistor 106. The terminal VDD and terminal VSS are provided by a power circuit (not shown). The OLED 102 and driving transistor 106 may be influenced by environmental temperature and manufacturing inaccuracy, and as a result, unstable illumination is induced in the pixel circuit.
FIG. 2 shows the relationships between conventional pixel brightness and temperature. The horizontal axis is temperature, and vertical axis a normalized value. The terminal VDD and terminal VSS are not influenced as temperature varies, but the brightness is proportional to the temperature. Temperature compensation is therefore desirable for driving the pixel circuit.